PHP writes null to JSON file - php

Question

I've seen similar posts to this question but I can't seem to figure it out. I have a small PHP script that reads and writes form input to a JSON file, like this –
$file = 'data.json';
$arr_data = array();
$formdata = array(
'name' => strip_tags( trim($_POST['formName']) ),
'email' => $email,
'phone' => strip_tags( trim($_POST['formPhone']) ),
'message' => strip_tags( trim($_POST['formMessage']) )
// also tested this just using reg strings
);
$jsondata = file_get_contents($file);
//var_dump($jsondata); returns whatever string content is in the file, so seems to work
$arr_data = json_decode($jsondata, true);
array_push($arr_data, $formdata);
//var_dump($arr_data); returns NULL, not sure what happens here
$jsondata = json_encode($arr_data, JSON_PRETTY_PRINT);
file_put_contents($file, $jsondata);
Any ideas? Using PHP 5.5.9, checked that files are writeable. Both files have UTF8 encoding.

Answer 1

json_decode() will return NULL if the input is blank. Try this to ensure your $arr_data is an array...
$arr_data = json_decode($jsondata, true);
if ($arr_data === null) {
$arr_data = [];
}

Answer 2

maybe you want this code
<?php
$file = 'data.json';
$email='your mail info';
$arr_data = array();
$formdata = array(
'name' => strip_tags( trim($_POST['formName']) ),
'email' => $email,
'phone' => strip_tags( trim($_POST['formPhone']) ),
'message' => strip_tags( trim($_POST['formMessage']) )
// also tested this just using reg strings
);
$jsondata = file_get_contents($file);
//var_dump($jsondata); returns whatever string content is in the file, so seems to work
$arr_data = json_decode($jsondata, true);
// I added for if data.json is null to empty array
$arr_data = is_null($arr_data)?array():$arr_data;
array_push($arr_data, $formdata);
//var_dump($arr_data); returns NULL, not sure what happens here
$jsondata = json_encode($arr_data, JSON_PRETTY_PRINT);
file_put_contents($file, $jsondata);
?>
others
you should change your code to keep post data not null or default values and pay attention to file_get_contents method about file length